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Elevated Heterotrophic Prokaryotic Production Supported by Low Nucleic Acid Prokaryotes at a Cyclonic Eddy Edge in the Northwest Pacific

Junyi Ni,Mingming Chen,Jiaming Shen,Li Liu,Zong-Pei Jiang,Kuanbo Zhou,Xiaolin Li,Minhan Dai,Yao Zhang

The cyclonic eddy uplifts nutrient-rich seawater into the euphotic zone, typically directly enhancing phytoplankton abundance and primary production. However, its impact on heterotrophic prokaryotic production (HPP) remains unclear due to the complex interplay of multiple indirect factors governing this process. Here, we conducted a comprehensive investigation of the distribution of picophytoplankton and heterotrophic prokaryotes, prokaryotic community structure, and HPP within a cyclonic eddy in the western North Pacific subtropical gyre. The results indicated that despite the higher abundance of picophytoplankton accompanied by nutrient upwelling at the eddy core compared to the edge, higher levels of HPP were observed at the eddy edge between 100 and 200 m, consistent with the distribution of the low nucleic acid content (LNA) prokaryotes. The significant positive correlation between HPP and the proportion of LNA group in total heterotrophic prokaryotes suggested a primary contribution from the LNA group over the high nucleic acid content (HNA) group. SAR11, a typical member of the LNA group, may primarily contribute to the elevated HPP observed at the eddy edge. The changes in temperature, nutrients, and light intensity induced by the cyclonic eddy may significantly influence the distribution and activity of HNA and LNA groups, potentially exerting a greater impact on HPP compared to phytoplankton-related factors. These findings contribute to understanding the underlying mechanisms of HPP responses to cyclonic eddies in the oligotrophic open ocean. Key Points Higher heterotrophic prokaryotic production (HPP) occurred at the eddy edge, while picophytoplankton predominantly occurred in the core The elevated HPP at the eddy edge was attributed to low nucleic acid prokaryotes, such as SAR11 The altered temperature, nutrients, and light intensity may impact the distribution and HPP in the eddy Plain Language Summary Mesoscale eddies are common features of global ocean circulation. Cyclonic eddies uplift isopycnals at their centers, bringing cold, nutrient-rich waters from the lower layers to the surface within the eddy, thereby promoting higher phytoplanktonic abundance and primary production typically observed at the eddy core. However, the response of heterotrophic prokaryotic distribution and heterotrophic prokaryotic production (HPP) to the upwelling in the cyclonic eddy remains unclear, as limited studies have not yet reached a consensus. We tracked a cyclonic eddy in the western North Pacific subtropical gyre to compare prokaryotic abundance, community composition, and HPP between eddy core and edge. The results indicate that, while picophytoplankton abundance was higher at the eddy core compared to the edge, HPP was predominantly elevated at the edge in response to the cyclonic eddy. The low nucleic acid content (LNA) prokaryotes dominated by SAR11 may significantly contribute to the increased HPP, as opposed to the traditionally assumed high nucleic acid content (HNA) group. The main factors influencing the distribution of LNA and HNA groups, as well as HPP, include temperature, nutrients, and light intensity altered by the eddy. This study advances the understanding of heterotrophic prokaryotic responses to cyclonic eddy dynamics.